The present invention relates to a plasma display panel driving method and a plasma display panel display apparatus used as the display screen for computers, televisions and the like, and in particular to a driving method which uses an address-display-period-separated sub-field (hereafter referred to as ADS) method.
Recently, plasma display panels (hereafter referred to as PDPS) have become the focus of attention for their ability to realize a large, slim and lightweight display apparatus for use in computers, televisions and the like.
PDPs can be broadly divided into two types: direct current (DC) and alternating current (AC). One example of a DC PDP is described in EPO 762,461, which discloses a PDP in which discharge cells are arranged in a matrix. AC PDPs are suitable for large-screen use and so are at present the dominant type.
High-definition television in which high resolutions of up to 1920xc3x971080 pixels is currently being introduced and PDPs should preferably be compatible with this kind of high-definition display, just as with other types of display.
FIG. 1 is a view of a conventional alternating current (AC) PDP.
In this PDP a front substrate 11 and a back substrate 12 are placed in parallel so as to face each other with a space in between. The edges of the substrates are then sealed.
Scanning electrode group 19a and sustain electrode group 19b are formed in parallel strips on the inward-facing surface of the front substrate 11. The electrode groups 19a and 19b are covered by a dielectric layer 17 composed of lead glass or similar. The surface of the dielectric layer 17 is then covered with a protective layer 18 of magnesium oxide (MgO). A data electrode group 14 formed in parallel strips is covered by an insulating layer 13 composed of lead glass or similar are placed on the inward-facing surface of the back substrate 12. Barrier ribs 15 are placed on top of the insulating layer 13, in parallel with the data electrode group 14. The space between the front substrate 11 and the back substrate 12 is divided into spaces of 100 to 200 microns by the barrier ribs 15. Discharge gas is sealed in these spaces. The pressure at which the discharge gas is enclosed is normally set below external (atmospheric) pressure, typically in a range of between 200 to 500 torr.
FIG. 2 shows an electrode matrix for the PDP. The electrode groups 19a and 19b are arranged at right angles to the data electrode group 14. Discharge cells are formed in the space between the substrates, at the points where the electrodes intersect. The barrier ribs 15 separate adjacent discharge cells preventing discharge diffusion between adjacent discharge cells so that a high resolution display can be achieved.
In monochrome PDPS, a gas mixture composed mainly of neon is used as the discharge gas, emitting visible light when discharge is performed. However, in a color PDP like the one in FIG. 1, a phosphor layer 16 composed of phosphors for the three primary colors red (R), green (G) and blue (B) is formed on the inner walls of the discharge cells, and a gas mixture composed mainly of xenon (such as neon/xenon or helium/xenon) is used as the discharge gas. Color display takes place by converting ultraviolet light generated by the discharge into visible light of various colors using the phosphor layer 16.
Discharge cells in this kind of PDP are fundamentally only capable of two display states, ON and OFF. Here, an ADS method in which one frame (one field) is divided into a plurality of sub-frames (sub-fields) and the ON and OFF states in each sub-frame are combined to express a gray scale is used.
FIG. 3 shows a division method for one frame when a 256-level gray scale is expressed. The horizontal axis shows time and the shaded parts show discharge sustain periods.
In the example division method shown in FIG. 3, one frame is made up of eight sub-frames. The ratios of the discharge sustain period for the sub-frames are set respectively at 1, 2, 4, 8, 16, 32, 64, and 128. These eight-bit binary combinations express a 256 gray scale. The NTSC (National Television System Committee) standard for television images stipulates a frame rate of 60 frames per second, so the time for one frame is set at 16.7 ms.
Each sub-frame is composed of the following sequence: a set-up period, a write period, a discharge sustain period and an erase period.
FIG. 4 is a time chart showing when pulses are applied to electrodes during one sub-frame in one related art.
In the set-up period, all the discharge cells are set-up by applying set-up pulses to all of the scan electrodes 19a. 
In the write period, data pulses are applied to selected data electrodes 14 while scan pulses are applied sequentially to the scan electrodes 19a. This causes a wall charge to accumulate in the cells to be ignited, writing one screen of pixel data.
In the discharge sustain period, a bulk pulse voltage is applied across the scan electrodes 19a and the sustain electrodes 19b, causing discharge to occur in the discharge cells where the wall charge has accumulated, and light to be emitted for a certain period.
In the erase period, narrow erase pulses are applied in bulk to the scan electrodes 19a, causing the wall charges in all of the discharge cells to be erased.
In the above driving method, light should normally only be emitted in the discharge sustain period and not in the set-up, write and erase periods. However, discharge occurring when set-up or erase pulses are applied causes the whole panel to emit light and contrast drops accordingly. Discharge occurring when the write pulses are applied also causes discharge cells to emit light, having a further detrimental effect on contrast. Consequently, there is a need to develop techniques for resolving these problems.
The above PDP driving method also should make the discharge sustain period in each frame as long as possible in order to improve luminance. Accordingly, the write pulses (scan pulses and data pulses) should preferably be as short as possible, so that writing can be performed at high speed.
High resolution PDPs have a large number of scan electrodes, so it is particularly desirable that the write pulses (scan pulses and data pulses) be narrow to enable driving to be performed at high speed.
However, in a conventional PDP, setting the write pulse narrowly causes write defects, lowering the quality of the image displayed.
If the voltage for the write pulse is high and the pulse narrow, writing may conceivably be performed at high speed without write defects. Normally, however, higher speed data drivers have lower ability to withstand voltage, so that it is difficult to realize a driving circuit which can write at both a high voltage and a high speed.
In the above PDP driving method, another important issue is driving the PDP with low power consumption. To achieve this, the inefficient power consumed in the discharge sustain period should be reduced to increase luminous efficiency.
An object of the present invention is to provide a PDP driving method that operates at high speed, and improves contrast without causing write defects. A further object of the present invention is to provide a PDP driving method that improves luminous efficiency. Yet another object of the present invention is to provide a PDP driving method that produces high image quality and high luminance without causing flicker and roughness on the screen.
In the present invention, a staircase waveform that rises in two steps or more is used for the set-up pulses. Using this kind of waveform for the set-up pulses rather than a simple rectangular pulse improves contrast without producing write defects.
Using a staircase waveform that falls in two steps or more for the write pulses rather than a simple rectangular pulse enables high speed driving to be performed without causing write defects.
Meanwhile, using a staircase waveform that rises in two steps or more for the write pulses improves contrast without causing write defects.
Furthermore, using a staircase waveform that falls in two steps or more rather than a simple rectangular waveform for the sustain pulses allows a high voltage to be set for the sustain pulses and ensures that operations are performed stably, so that high image quality can be realized.
If a staircase waveform that rises in two steps or more is used for the sustain pulses rather than a simple rectangular wave, luminous efficiency is improved. A particularly marked improvement in luminous efficiency is achieved when the second step of the rising portion and the first step of the falling portion of the waveform correspond to a continuous function.
Luminous efficiency may also be improved by using a waveform whose rising portion is a slope for the sustain pulses.
Another way of improving luminous efficiency is using a waveform in which the voltage at a time when the discharge current is highest is higher than the applied voltage occurring at a time when the pulse starts for the sustain pulses.
Using a staircase waveform with two or more steps for the first sustain pulse to be applied during the discharge sustain period improves image quality.
Additionally, using a staircase waveform that rises in two steps or more for the erase pulses rather than a simple rectangular waveform improves contrast and enables a high quality image to be realized.
Using a staircase waveform that falls in two or more steps for the erase pulses shortens the erase period.
These effects can be further enhanced by using staircase waveforms for the set-up, write, sustain and erase pulses simultaneously.
Staircase waveforms that rise and fall in two steps, like the ones described as being used for the set-up, write, sustain and erase pulses, are realized by adding two or more pulses together.